Method of and device for manufacturing substituted single crystals

ABSTRACT

The crystal growth is carried out in a crucible which is rotatable about its horizontal axis. Initially, the melt is not in contact with an extra quantity to be added of the components of the single crystals to be manufactured, nor with a seed crystal, if any. After rotating the crucible about its horizontal axis, the melt is in contact with the extra quantity to be added of the components and with the seed crystal, if any. Crystallization is terminated by rotating the crucible again about its horizontal axis.

United States Patent 11 1 Tolksdorf et al.

[ Mar. 25, 1975 METHOD OF AND DEVICE FOR MANUFACTURING SUBSTITUTED SINGLE CRYSTALS [75] Inventors: Wolfgang Tolksdorf, Tornesch; Fritz Welz, Hamburg, both of Germany [73] Assignee: U.S. Philips Corporation, New

York, NY.

221 Filed: Feb. 20, 1973 21 Appl. No.: 334,033

52 us. Cl. 252/6257, 23/301 SP, 23/294, 23/300, 23/305 511 Int. Cl. B0lj 17/20, C04b 35/40 [58] Field of Search 23/301 R, 301 SP, 273 R, 23/273 U, 273 F, 273 SD, 295, 299, 300; 252/6257, 62.56

[56] References Cited UNITED STATES PATENTS 3,335,084 8/1967 Hall 23/295 Desoerulion T 3,444,084 5/1969 Geller et a1. 252/6257 3,479,292 11/1969 Chegwidden et a1. 252/6257 3,496,108 2/1970 Kolb et a1. 252/6257 3,677,712 7/1972 Tolksdorf 23/273 SP 3,697,320 10/1972 Hiskes 252/6257 Primary Examiner-Norman Yudkoff Assistant Examiner-D. Sanders Aztorney, Agent, or Firm-Frank R. Trifari [57] ABSTRACT The crystal growth is carried out in a crucible which is rotatable about its horizontal axis. Initially, the melt is not in contact with an extra quantity to be added of the components of the single crystals to be manufactured, nor with a seed crystal, if any. After rotating the crucible about its horizontal axis, the melt is in contact with the extra quantity to be added of the components and with the seed crystal, if any.

Crystallization is terminated by rotating the crucible again about its horizontal axis.

2 Claims, 7 Drawing Figures 7 Heater Elements 20 Cooling Finger 2 Thermo Elements PATENTEDHAY25|975 3.873.463

sum 1 5 5 7 Heater Elements 20 Cooling Finger 2 Th rmo ,1

Elements PATENIED HAR 2 5 I975 sum 5. 9 5

500 660 S00E10 500%0 mM V V V i V C D E F Fig.7

METHOD OF AND DEVICE FOR MANUFACTURING SUBSTITUTED SINGLE CRYSTALS The invention relates to a method of manufacturing substituted single crystals by crystallisation from a liquid solution of the components of the crystal material in a crucible, if desired with the use of a seed crystal, in which during the crystallisation a temperature drop is adjusted and maintained in the liquid solution and quantities of the components are constantly added to the liquid solution. The invention furthermore relates to a method for carrying out the method.

Single crystals are used as essential components in many devices for natural science, medical science and industry. For example, single crystals having optimum magnetic, electric, optic and acoustic properties are necessary for the manufacture for semiconductors and lasers and also for microwave components and light modulators. In many fields of application, for example, in microwave filters, there exists a need not only for single crystals which consist of a single chemical substance, but also for mixed single crystals or substituted single crystals. For example, the saturation magnetisation (4 11' M.) ofYttrium-iron-garnet (Y Fe O can be reduced by replacing a part of the iron ions by nonmagnetic ions, for example, gallium ions or aluminium ions. In order to obtain useful single crystals of this type it is very important that the content of gallium and aluminium, respectively, be constant and adjustable over a large crystal range.

The manufacture of single crystals, in particular garnet crystals, by slow cooling of a liquid solution of garnet in a crucible and separating the resulting crystals from the still liquid residual melt by rotating the crucible is known (British Patent specification l,223,236). In this manner, however, no mixed crystalshaving a homogeneous composition can in principle be obtained when the ratio of the mixing components in the solution and in the crystal are different. Said ratio will also vary generally with the temperature. Experiments have already become known to reduce said influences by suitable compositions of the melt, (H. S. Peiser (editor), Crystal Growth," Oxford 1967, pp. 457 461) or by elevated gas pressure (Journal Crystal Growth Vol. 3, 4(1968) pp. 452 454).

Furthermore, a method is known of manufacturing single crystals of a material by crystallisation from a solution of the material in an auxiliary melt in which the solution. at a concentration of the dissolved material which initially is not higher than the saturation concentration at the temperature of the coolest zone, is subjected to a temperature gradient and in which extra dissolved material is continuously added, while the temperature gradient is maintained, as a result of which crystallisation and progressing crystal growth of the dissolved material is produced in the coolest zone. Small quantities of additions can be incorporated in the crystals (British Patent specification 952,385). This method also is not suitable for the manufacture of mixed crystals having a homogeneous composition, which appears from the fact that the additions are present only in the initial melt and not in the extra added material and that, except for the natural convection, no possibility exists for adjusting the equilibrium.

A principle cause of the inhomogeneity of the content of substituents of the mixed crystal lies in the so far LII used methods themselves. According to the Distribution Law of Nernst the concentration in the crystal and in the melt varies proportionally with the weight of the growing mixed crystal when the distribution coefficient is no-tequal to'one.

It is the object of the invention to counteract said effect and to manufacture substituted single crystals in which the substituent is homogeneously distributed and which thus show a constant ratio of the mixing components.

According to the invention, prior to the beginning of the crystallisation, the solution is heated at a temperature above the point of the spontaneous nucleation, the solution is slowly cooled and, after spontaneous nucleation has taken place, simultaneously:

a. the warmest zone of the solution is contacted with the quantity of the components to be added, and

b. a seed crystal is introduced into the coolest zone of the solution, if desired, by rotation of the crucible about its horizontal axis, the crystallisation is localized by cooling and the increasing insulating action of the growing crystal is compensated for and the crystallisation is terminated in that the liquid solution is separated from the crystals formed by rotation of the crucible about its horizontal axis.

During the heating at a temperature above the point of the spontaneous nucleation, the crucible is preferably rotated about its vertical axis, the speed of rotation being constantly and alternately increased and decreased, and this is continued during the crystallisation. It is to be noted that it is known to obtain a good mixing of the crystal melts by alternately increased and decreased rotation of the crystallisation vessel about its horizontal axis (Journal Crystal Growth, Vol.8 (197 l p. 304).

The device for carrying out the method according to the invention is characterized in that the crucible containing the liquid solution is closed on all sides and is transferred to a furnace, that the crucible is rotatable about a shaft parallel to the surface of the liquid solution, that a desaeration tube is provided in the part of the crucible which is uppermost in the starting position, said tube being bent downwards and having such a length that in the rotated condition no liquid solution can flow away, that a seed crystal is possibly present in the crucible in the starting position above the liquid solution, that the quantity of the components to be added is present in a second crucible which is provided with apertures and is arranged inside the first crucible in such manner that the quantity of the components to be added can contact the liquid solution through the'apertures only after rotation of the first crucible in the crystallisation position and that a cooling finger extends in the furnace in such manner as to touch the wall of the first crucible in the crystallisation position in a location where the seed crystal, if any, is present.

In particular, the crucible is arranged in the furnace so as to be rotatable about a shaft which extends vertical with respect to the surface of the liquid solution.

In the method according to the invention, the crystallisation takes place in a temperature gradient, which is known per se. While the crucible of the device according to the invention is in the starting position, the liquid solution is saturated by cooling without contacting the extra material to be added and the seed crystal. Only upon rotation of the crucible about its horizontal axis in the crystallisation position is the saturated solution contacted with the extra material to be added and the seed crystal. During said rotation, the crystals formed upon cooling the liquid solution by spontaneous nucleation are simultaneously removed from the solution. This process can be carriedout within the existence range of the material to be crystallised at arbitrary temperatures. The isolating effect of the growing crystal is compensated for by continuous variation of the temperature gradient by means of a local air cooler (cooling finger). After termination of the crystallisation process the grown crystal is removed from the liquid solution by rotation of the crystallisation vessel about its horizontal axis. Then the crucible is again in its starting position. In this manner, in the method according to the invention, and possibly starting from a seed crystal, a uniform substitution, for example of a part of the iron by gallium in yttrium-iron-garnet, is achieved and a homogeneous mixed crystal is thus obtained isothermally by constant growth temperature in a temperature gradient, that is to say, without a cooling program, with a constant ratio of the mixing components by a good mixing of the liquid solution in the presence of a polycrystalline material to be added.

An embodiment of the invention is shown in the drawing and will be described in greater detail hereinafter with reference to experiments.

FIG. 1 shows the furnace with the crystallization vessel arranged therein,

FIG. 2 shows the crystallization vessel in the starting position,

FIG. 3 shows the crystallization vessel during the crystallization,

FIG. 4 shows the final position of the crystallization vessel which corresponds to the starting position,

FIGS. 5 and 6 show divided mixed crystals obtained in the experiments to be described hereinafter in which the measured ranges of the analysis results recorded in Tables 1 and 2 are stated,

FIG. 7 is a graphical representation of the saturation magnetisation (4 1T M,,) measured in mixed crystals manufactured according to the invention.

FIG. 1 shows a diagrammatic arrangement of the furnace used in the experiments to be described hereinafter. The furnace is arranged in an outer metal screen 1. Between said screen and the insulation of the furnace an air space with air inlets 2 and exhaust device 3 is saved. The insulation of the furnace consists of layers of compressed asbestos 4, insulation fibres 5 and insulation stones 6 succeeding each other from the inside to the outside. The furnace comprises heater elements 7, for example, of silicon carbide, and thermo-elements 8. The crystallization vessel, i.e., the crucible 9, is arranged in the interior space of the furnace by means of a holder 10, for example, of aluminium oxide, which is lined with insulation fibres 11.

The crucible 9 is shown in FIG. 1 without the details of its inner construction (these details are shown in FIGS. 2 to 4). The crucible 9 is rotatable about a shaft 13 which is vertical with respect to the surface of the liquid solution 12 and furthermore swingable or tiltable about a second shaft 14 which is parallel to the surface of the liquid solution 12, it being possible for the two shafts l3 and 14 to be separated from the crucible 9 so that the crucible 9.can rotate, for example, about the vertical shaft 13 while the horizontal shaft 14 is removed, and conversely. On the outside of the crucible 9 a desaeration tube 15 is provided in the half of the crucible 9 which is uppermost in the starting position. The desaeration tube 15 is bent downwards and has such a length that in the rotated condition, that is to say in the crystallization position (FIG. 3) of the crucible 9, no liquid solution can flow out of it.

In FIGS. 2 to 4, the crucible (the crystallization vessel) which contains the liquid solution 12, is closed on all sides and is arranged in the furnace (not shown), is also denoted by 9. In the starting position (FIG. 2) the seed crystal 16 is arranged in the crucible 9 above the liquid solution 12. The additional quantity of the mixture to be crystallized, so the nutrient 17, is present in a second crucible 18 which has aperture 19 and is arranged inside the first crucible 9 in such manner that the nutrient 17 can contact the liquid solution 12 via the apertures 19 only after rotation of the first crucible 9 in the crystallization position (FIG. 3). The cooling finger 20 extending in the furnace (FIG. 1) contacts the wall of the crucible 9 in the crystallization position (FIG. 3) at the location where the crystal 16 is present.

The following experiments were carried out with such a device:

EXPERIMENT l 900 gr of a melt of the composition were provided in the crystallization vessel in the starting position (FIG. 2) at l,l00C. The crystallization vessel consisted of a platinum crucible 9 (diameter mm, height mm) in which an inner crucible 18 (diameter 2| mm, height 50 mm) was welded on the bottom. 33 gr. of additional mixture to be crystallized, so nutrient, in the form of a sintered body 17 of polycrystalline garnet were provided in the inner crucible 18. The nominal composition of the garnet in the melt l2 and in the nutrient 17 were Y Fe Ga 8 12- After filling the sintered body 17 the inner crucible 18 was closed with a platinum cover 19 in the form of a sieve. The pressure compensation during heating was enabled by a tube 15 welded in the cover of the crucible 9. The seed crystal 16 (0.5 gr) of unsubstituted yttrium-iron-garnet was also secured to the cover. In the starting position (FIG. 2) the crucible 9 was heated at 1,280C for 2 hours for homogenizing the melt 12. The mixing was stimulated by constant acceleration and deceleration of the speed of rotation of the crucible. (rotation about the shaft 13). The number of revolutions was raised from 20 to 80 rpm. within 15 seconds and subsequently reduced in the same manner.

After the spontaneous nucleation, the melt was maintained at a temperature of 1,085C for 24 hours for adjusting the equilibrium. The crystallization vessel was then tilted 180 about its horizontal shaft 14 in the crystallization position (FIG. 3). The seed crystal 16 was introduced into the melt 12. Simultaneously, the crystals formed by spontaneous nucleation were removed from the melt.

After a constant temperature variation the weekend over (68 hours), the seed crystal 16 was cooled by the cooling finger 20. The flow rate of the cooling air was slowly increased so that the temperature of the thermoelement 8 (FIG. 2) was reduced by 04C per hour to l,07lC.

By means of the variable rotation of the crucible as described, the transport of material from the sintered body 17 into the melt 12 and to the seed crystal 16 was stimulated. After a further constant temperature variation for 6 days without increasing the flow of cooling air the crystallization was terminated by tilting the crucible in the starting position (FIG. 4). The weight of the seed crystal had increased from 0.5 gr. to 6 gr.

EXPERIMENT 2 The residual melt of experiment 1 was brought again at the initial composition by the addition of the quantities of oxides corresponding to the removed crystal. As a seed crystal 16 was used an unsubstituted yttriumiron-garnet single crystal. The homogenization of the melt corresponded to that of experiment 1. The seed crystal 16 was rotated in the melt 12 at l,O85C. After having cooled the bottom of the crucible with the seed crystal by approximately 0.4C per hour to l,074C by increasing cooling air, it was tried, by a heating rate of 10C per hour to 1,084C, to dissolve any undesired crystals formed. The crystallization temperature was then adjusted by the cooling air in accordance with the preceding increase to l,069C. The mixing of the melt by variable rotation was stimulated by the lateral incorporation of a stirrer 21.

After a constant temperature variation for 4 days, the crystallization was terminated. The weight of the seed crystal had increased from 0.5 g to I4 g.

The crystal of experiment 1 was divided as shown in FIG. 5. By means of an X-ray fluorescence analysis, while using densely sintered polycrystalline galliumsubstituted garnets as assay material, the gallium analysis was carried outwith a diaphragm of 3.5 mm diameter on the polished cutting faces of the segments Tl/l, Tl/2, Tl/3 and T2b. The results for the measured ranges shown in FIG. 5 are plotted in Table l. The average value was I 0.76 (for Y3Fe5 rGa O12). The maximum deviations, with an analysis error of less than i 0.01 were i 0.03, in which in the centre of the crystal the value within the analysis accuracy corresponded to the average value.

The crystal of experiment 2 was divided as shown in FIG. 6. The measured ranges and analysis results may be derived from FIG. 6. and Table 2. The average gallium content was found at T=O.77 d: 0.01;,. For all the investigated segments said value was only slightly outside the analysis error, in the greater part of the crystal even inside the analysis error.

In order to measure the saturation magnetization (4 7T M,,) as room temperature, balls of 0.7 mm diameter were manufactured from the crystal of experiment 1, segment T1/2. The same was done with experiment 2, T1 and T2, in which each case each time the centre and the edge region of the crystal plates were processed separately. The saturation magnetisation could be determined in the balls from the frequency difference between the 210 and the Walker mode of the magnetic reasonance. The results are shown in FIG. 7. The horizontal axis gives the saturation magnetisation 4 1r M, in Gauss; A relates to a crystal not manufactured according to the invention, i.e., without addition of nutrient, B relates to experiment 1 Tl/2, C to experiment 2 T1 edge, D to experiment 2 T1 centre, E to experiment 2 T2 edge and F to experiment 2 T2 centre. The vertical axis gives the number of balls K. For comparison are stated the measured values of a crystal segment which was obtained by cooling the liquid solution without nutrient.

The quality factor Q of the microwave resonator in the case of balls from crystals manufactured according to the method described is on an average considerably higher than in the case of balls according to the known method. All segments used were without inclusions of the solvent.

The method according to the invention enables the crystallisation of mixed crystals from the liquid solution which have a nearly constant distribution of the mixing components. The still small spreading of the gallium content in the crystals of experiment 2 is to be ascribed to the stronger mixing of the melt and better rinsing of the nutrient as a result of the lateral plate on the reaction vessel. From this segment 40 balls could be manufactured having 4 1r M -values lying between 525 and 535 Gauss, the gallium content x in the formula Y Fe Ga,O, being 0.77 within the analysis error limit of d: 0.01.

What is claimed is:

1. A method of manufacturing substituted single crystals by crystallization from a liquid solution of the components of the crystal material in a crucible, if desired with the use ofa seed crystal, in which during the crystallisation a temperature drop is adjusted and maintained in the liquid solution and quantities of the components are constantly added to the liquid solution, characterized in that, prior to the beginning of the crystallization, the solution is heated at a temperature above the point of the spontaneous nucleation then the solution is slowly cooled and, after the spontaneous nucleation has taken place, simultaneously a. the warmest zone of the solution is contacted with the quantity of the components to be added. and

b. a seed crystal is introduced into the coolest zone of the solution, if desired,

by rotation of the crucible about its horizontal axis, that the crystallization is localized by cooling and the increasing insulating action of the growing crystal is compensated for and that the crystallization is terminated in that the liquid solution is separated from the crystals formed by rotation of the crucible about its horizontal 

1. A METHOD OF MAUNFACTURING SUBSTITUTED SINGLE CRYSTALS BY CRYSTALLIZATION FROM A LIQUID SOLUTION OF THE COMPONENTS OF THE CRYSTAL MATERIAL IN A CRUCIBLE, IF DESIRED WITH THE USE OF A SEED CRYSTAL, IN WHICH DURING THE CRYSTALLISATION A TEMPERATURE DROP IS ADJUSTED AND MAINTAINED IN THE LIQUID SOLUTION AND QUANTITIES OF THE COMPONENTS ARE CONSTANTLY ADDED TO THE LIQUID SOLUTION, CHACTERIZED IN THAT, PRIOR TO THE BEGINNING OF THE CRYSTALLIZATION, THE SOLUTION IS HEATED AT A TEMPERATURE ABOVE THE POINT OF THE SPONTANEOUS NUCLEATION THEN THE SOLUTION IS SLOWLY COOLED AND, AFTER SPONTANEOUS NUCLEATION HAS TAKEN PLACE, SIMULTANEOUSLY A. THE WARMEST ZONE OF THE SOLUTION IS CONTACTED WITH THE QUANTITY OF THE COMPONENTS TO BE ADDED, AND B. A SEED CRYSTAL IS INTRODUCED INTO THE COOLEST ZONE OF THE SOLUTION, IF DESIRED, BY ROTATION OF THE CRUCIBLE ABOUT ITS HORIZONTAL AXIS, THAT THE CRYSTALLIZATION IS LOCALIZED BY COOLING AND THE INCREASING INSULATING ACTION OF THE GROWING CRYSTAL IS COMPENSATED FOR AND THAT THE CRYSTALLIZATION IS TERMINATED IN THAT THE LIQUID SOLUTION IS SEPARATED FROM THE CRYSTALS FORMED BY ROTATION OF THE CRUCIBLE ABOUT ITS HORIZONTAL AXIS.
 2. A method as claimed in claim 1, characterized in that during the heating at a temperature above the point of the spontaneous nucleation, the crucible is rotated about its vertical axis, the speed of rotation being constantly and alternately increased and decreased, and this is continued during the crystallization. 